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Title: ABIOTIC TRANSFORMATION OF CHLORPYRIFOS TO CHLORPYRIFOS OXON IN CHLORINATED WATER

Author
item WU, JIGANG - VA STATE UNIV
item Laird, David

Submitted to: Environmental Toxicology and Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/12/2002
Publication Date: 2/3/2003
Citation: WU, J., LAIRD, D.A. ABIOTIC TRANSFORMATION OF CHLORPYRIFOS TO CHLORPYRIFOS OXON IN CHLORINATED WATER. ENVIRONMENTAL TOXICOLOGY AND CHEMISTRY. 2003. V. 22(2). P. 261-264.

Interpretive Summary: Chlorpyrifos is an insecticide that was widely used to control insect pests in households for several decades. Recently, new concerns about the safety of chlorpyrifos prompted the EPA to initiate a phase out of most chlorpyrifos products intended for domestic use. One of the reasons chlorpyrifos was long considered a relatively safe insecticide is that it must be first converted to chlorpyrifos oxon by liver enzymes before it is acutely toxic to humans and animals. We discovered that the chlorine in chlorinated tap water will rapidly transform chlorpyrifos to chlorpyrifos oxon. This means that there is a risk that humans and pets may inadvertently be exposed directly to chlorpyrifos oxon when chlorpyrifos products that have been mixed with tap water are used. This information will be of value to the EPA and other action agencies as they seek to evaluate the risk associated with various uses of chlorpyrifos products.

Technical Abstract: In vivo transformation of chlorpyrifos to chlorpyrifos oxon is believed to be a prerequisite for this insecticide to display acute toxicity to organisms. We discovered that active chlorine dispersed in water causes the rapid abiotic transformation of chlorpyrifos to chlorpyrifos oxon. The proposed mechanism for the transformation is an electrophilic attack by HOCl on the thion (P=S) double bond of chlorpyrifos followed by desulfuration of chlorpyrifos to its oxon analog. The pH and temperature of a Cl-aqueous solution controls the speciation of HOCl and OCl**- in water and thereby influence the transformation process. Chlorpyrifos oxon is a potent anticholinesterase that is about 1000 times more toxic than chlorpyrifos. Because water chlorination is commonly used for treatment of domestic water supplies, the findings raise a new concern about the safety of domestic use of chlorpyrifos products.